Speed, and distance, particularly, revolution detection apparatus are increasingly required for the detection of rotary speed or positions, e.g. angular position of a rotating body, for open and closed-loop control of engines. Such apparatus may function according to the eddy-current measurement method, known per se, in which upon the approach of the electrically conductive zones to a measuring transducer having alternating current of high frequency flowing through it, eddy currents are generated. The eddy currents have the effect of energy extraction, that is, a reduction in amplitude of the electrical oscillation in the measuring transducer. This is made use of, after approximate processing, as a measuring signal for the speed, e.g. rotary speed, or for the position of the moving body.
In a known apparatus of this kind, which is applied in particular to rotating engines, gears which are already present in the engines (for instance the flywheel gear in the combustion engine) are utilized with a suitable measuring transducer for obtaining pulses. The tooth crests represent the zones of the electrically conductive material. Difficulties arise as a result of the substantial dependency of the measuring transducer signal upon the size of the measuring transducer and the distance of the measuring transducer from the moving body, in this case the rotating gear. In order to attain high resolutions for an exact measurement of the position or speed, large numbers of teeth must be detected with small dimensions on the part of the measuring transducer. The measuring transducer must have geometric dimensions, such as its diameter, which correspond approximately to the dimensions of the teeth, so as to be able to reliably detect individual teeth. However, three limitations are placed on the reduction of the dimensions of the measuring transducer: First, the manufacture of minimum-size measuring transducers is expensive; second, the measuring distance between the measuring transducer and the electrically conductive zones must necessarily be reduced; and, finally, there is an increasing reduction of the measuring signal obtained as the measuring transducers become smaller. This means that in many cases it is no longer possible at all to manufacture measuring transducers capable of detecting the finest gear teeth pitch, having tooth widths of less than 1 mm.
Even where only coarse pitch gears are used, such as the flywheel gears in combustion engines, detection, problems arise. Because of the high tolerances in the radial direction and as a result of severe mechanical oscillations on the part of the rotating flywheel during operation, the measuring transducer must be at a relatively long measuring distance from the electrically conductive zones, in this case the teeth to be detected. The consequence is that the measuring transducer, if it is still to be capable of emitting a sufficiently large measuring signal, must have a relatively large diameter; as a result, it will generally already detect several teeth at once, and clean resolution of the individual tooth becomes difficult.